The present invention relates to distance amplitude compensation techniques used in connection with ultrasonic pulse echo testing for flaws and discontinuities in a workpiece.
The use of ultrasonic pulse echo techniques to test workpieces is well known. High frequency electrical pulses are applied to a transducer which converts them into ultrasonic pulses which are applied to an entering surface of a workpiece to be tested. The ultrasonic pulses are reflected back towards the entering surface of the workpiece by discontinuities such as defects within the workpiece. The same, or a different, transducer receives the reflected pulses and reconverts them into electrical pulses. The electrical pulses may be displayed on a cathode ray tube. The above-described testing procedure is further explained in U.S. Pat. No. 2,280,226 issued to F. A. Firestone on Apr. 21, 1942.
If one or more test blocks are provided with identically sized reflectors spaced apart and at varying depths therein, the amplitudes of the displayed electrical pulses are initially relatively small, increase in magnitude to a maximum value as the depth of the reflectors within the workpiece increases, and then decrease in amplitude as the depth of the reflectors within the workpiece is still further increased. Thus, the magnitudes of the reflected pulses are dependent upon both the sizes of the corresponding reflectors and the depths of the reflectors within the workpiece. (The term "depth," as used above, is intended to mean the distance between the entrance surface and the reflector within the workpiece).
The reason for the above described phenomenon in which the magnitude of a reflected pulse is dependent upon both the size of the reflector and its depth within the workpiece is due to the "near field" and "far field" effects as well as attenuation in the material. The "near field zone" is the portion of the workpiece closest to the entrance surface in which the reflected pulses increase in size. The maximum amplitude of the reflected pulse is realized at a depth referred to as the "near field limit". As the depth of the reflector is further increased beyond the near field limit and into the "far field", the magnitudes of the reflected pulses progressively decrease due to the diverging beam and to attenuation by the test material. The near field and far field effects of a testpiece are discussed in U.S. Pat. Nos. 3,033,029 and 4,056,971.
In practice, only the pulse echo from a single reflector (or test hole) can be displayed at any one location of the transducer. Thus, only one point on the composite curve representing the echo signals can be displayed on the cathode ray tube at any given instant of time. The pulse echo displayed on the cathode ray tube at any given instant corresponds to the single test hole which is reflecting the applied ultrasonic energy at that instant.
One form of a distance amplitude compensation system varies the gain of the receiver (which receives the reflected pulse echoes from a testpiece) to compensate for the above-described near field and far field effects. The objective of a distance amplitude compensation system is to assure that all pulse echoes from identically sized defects are displayed as equal magnitudes, irrespective of the depth of the defects within the workpiece. The amplitudes of all displayed echoes are then functions of only the size of their corresponding defects.
A known disadvantage of conventional distance amplitude compensation systems is that complicated trial and error techniques are required to adjust the gain of the receiver to eliminate the near field and far field effect factors from the reflected pulse echoes. Since only one pulse echo from the test holes of a test block can be displayed on a cathode ray tube at any instant of time, many passes or scans of the transducer over each test hole are required to make an optimum adjustment of the magnitude of the echo. In practice, it has also been found that adjustment of one pulse echo affects the magnitudes of adjacent echoes, thereby requiring many additional scans and adjustments to provide optimum compensation for all pulse echoes. Equalization of the pulse echoes by varying the receiver gain with time using such trial and error techniques is time consuming, laborious and subject to errors.
An example of a conventional distance amplitude compensation system is disclosed in U.S. Pat. No. 3,033,029 (Weighart). Other conventional systems are known to those skilled in the art.
U.S. Pat. No. 4,056,971 describes a system which does not involve varying the receiver gain with time and thus avoids this aspect of the set-up problem for the curveshape generator. It does not compensate for the near field and far field effect of the workpiece by equalizing the displayed signals reflected from a given sized defect irrespective of the depth of the defect. Instead, this patent discloses a system which compares a reference distance amplitude response curve (of uncompensated reflected pulse signals from a given size defect at various depths within a test piece) with the uncompensated reflected signals of the workpiece being tested. If the signals from the workpiece under test exceed the reference signal, an alarm is actuated.
It is the main object of the present invention to provide a distance amplitude compensation system which enables an operator of the system to readily equalize the magnitudes of reflected pulse signals corresponding to given size defects at predetermined levels by varying the gain of the receiver of the system in accordance with a predetermined relationship, thereby eliminating the trial and error techniques of conventional systems.